Infrared Irradiation of H₂O:CO₂ Ice: A Combined Experimental and Computational Study of the Dissipation of CO₂ Vibrational Excitations

In interstellar ices, the ice matrix can have a great influence on the chemical reactions. The hydrogen-bonding network in pure water ices facilitates fast energy dissipation that, for example, stabilizes the HOCO complex, a crucial step in the formation of CO₂. To better understand the energy dynamics and its possible influence on the processes in the ice, we investigated a H₂O:CO₂ 1:4 ice mixture exposed to infrared irradiation on-resonance with the CO₂ vibrations. Experimentally, we find changes in the OH stretch of H₂O after irradiating the asymmetric stretch of CO₂ for several minutes with the intense monochromatic light of the FELIX free electron lasers. Using molecular dynamics simulations, we found that an excitation of the asymmetric stretch of CO₂ readily dissipates to other asymmetric stretches in the environment, but only dissipates to the CO₂ libration and H₂O twist modes after roughly 2 ns because of its minimal anharmonicity and coupling with other modes. This is significantly longer than the off-time between laser pulses of 1 ns, suggesting ladder climbing or that the stacking of the excitation boosts the experimentally observed changes. For infrared excitation of the CO₂ bending vibration, the simulations reveal a fast distribution of energy and coupling to the intermolecular interactions that lead to thermal heating of the H₂O vibrational modes. This is not observed on the time scale of the experiments. Still, both simulations and experiments reveal nonthermal annealing of the H₂O component of the mixed ice when exposed to infrared irradiation on-resonance with the CO₂ vibrations.